I saw that thing on Unbox Therapy. Two screens and no buttons? Fail.
Seriously, gaming on a touchscreen with on-screen buttons is about as uncomfortable as sleeping in sub-zero temperatures in your underwear. (I'm terrible at analogies, but you get the point.) Some physical buttons on that attachment would work wonders. Twin joysticks, face and shoulder buttons, a good D-pad, etc.
Seriously, I'm just not a fan of the design. It just looks so clunky and a pain in the ass to use, like an overpowered yet poorly thought out 2DS. Reminds me of the Nvidia SHIELD portable, which I don't think did that well. (At last that had physical controls.)
However, I'd love to see the horsepower on that phone. I hope it's good for Dolphin, it'd be nice to play Zelda or Melee in a portable form factor... with shitty on-screen controls... yay...
Yeah, I wouldn't recommend that phone to anyone who likes tactile controls. At least not without making them carry around a MOGA controller or something wherever they go. I'm just surprised companies want to make gaming phones, there's not really a market for them. Why would you need enhanced graphics processors and liquid cooling in your cellphone? In order to play shitty mobile games in 4K or something? (Insert obligatory Fortnite joke here)
This thing would be better used for things like on-the-go video editing and stuff like that, things that require good horsepower but can get away with touchscreen controls. (And video editing needs plenty of extra screen real estate, I speak from experience.)

In short, I'm not gonna buy this monstrosity, but I can see the appeal.